Modem commercial aircraft are designed for maximizing the dollar value of the payload, which can involve maximizing the passenger carrying capacity, taking into consideration passenger amenities which may command a premium fare.
For many passenger aircraft of circular or ovoid cross-section, the main cabin floor is disposed somewhat below the geometric center to provide a large passenger compartment having the usual ceiling, overhead storage compartments, and other amenities. Ducts, cables, safety equipment, etc., can run through the small "crown" at the top above the ceiling and overhead bins. The space below the cabin floor, substantially smaller than the cabin area itself, is designed for standardized cargo/baggage containers, as well as equipment bays, structural members, and auxiliary equipment. The area above the passenger cabin ceiling is even smaller. All in all, the goal is to provide a single, large, attractive passenger cabin with few encumbrances not directly associated with passenger comfort, such as galleys and lavatories.
Long range subsonic airliners may have non-stop flights of a duration of 12 hours or more. On such long flights, it is required by federal regulations to provide room for pilot rest areas and necessary to also provide attendant rest areas. For example, in the case of a Boeing 747, a small aft portion may be reserved for an upstairs bunkhouse, in an area aft of passenger seating, and it has been proposed to provide bunks in other non-seating areas, over or adjacent to other cabin monuments such as galley areas and/or areas adjacent to rear exit doors.